The only time that a joint monarchy in GB is recognized was that of William III and his wife Mary II 1689-94. Yet there have been a number of other instances that the history books ignore.
In 1170 Henry, son and heir of Henry II of England was crowned King to reign jointly with his father and although he died young in 1183 he was still King, so why isn't he Henry III?
In 1554 Philip II of Spain married Mary I of England and was recognized as King of England by Act of Parliament, so why do we not refer to him as King of England reigning from 1554 to 1598? The Act was not formally repealed until 1863.
In 1565 Mary, Queen of Scots married Henry, Lord Darnley and he was proclaimed joint-King, so why don't we know him as Henry, King of Scots?

Hmmm, good questions. I would imagine there are very good constitutional/legal reasons for these apparent aberrations, but I don't know what they are. However I can offer the following points.

Firstly the co-reign of William and Mary - one can call it a "Glorious Revolution" but I see it basically as a military coup backed up by a foreign invasion, albeit that the new man, William, was invited in by the legitimate government. William took the throne principally by right of conquest (albeit that the sitting head of state, James II, simply abandoned the country without offering much opposition and ran away to France) and backed legally by his wife's right of succession. Accordingly they ruled as co-regents (and lets not forget this was almost entirely by the grace of Parliament).

Now the case of Philip and Mary. Under the terms of the 1554 Act for the Marriage of Queen Mary to Philip of Spain, Philip was to enjoy Mary I's titles and honours for as long as their marriage should last, and indeed they were joint signatories for all legislation and by Act of Parliament Philip was termed King of England (and Ireland, France etc.) However when Mary died in 1558 Philip lost all his rights to the English throne and ceased to be King of England, Ireland and (as claimed) France, since all these titles were only held through his marriage to Mary and not in his own right. I suggest therefore that Philip is not usually counted as a king of England, not because he wasn't at some time the legitimate king, but because he didn't die as one (and also because in the national narrative as it came to be written, he was the villain who tried to invade Gloriana's "royal throne of kings, this sceptered isle, This earth of majesty, this seat of Mars, This other Eden, demi-paradise, ..." etc.)

Now Henry Lord Darnley of Scotland. I thought one of the principal reasons why he had such a strop (to put it mildly) was because while he was acknowledged as King Consort of Scotland (indisputable as he was married to the Queen) nevertheless Mary absolutely refused to grant him the Crown Matrimonial, which would have made him the successor to the throne if she died childless. He was therefore king in name while she was alive but not in any substance and again he died before her.

And finally Henry's II's son, Henry (imaginative lot those Angevins), while he was crowned as heir and supposedly joint ruler with his father, he is probably not recognised as a king because his father was still very much alive, firmly in charge, and refused to grant his co-regent son any meaningful independent power or responsibility. So again I guess its because Henry-the-young-king died while his father was still very much on the throne and in command.

Well that's how I see it.

PS - There's also Louis of France and potentially Louis I of England (although he never actually got crowned). During the rebellion by English barons against the unpopular King John in the First Barons' War, the throne was offered to Prince Louis (son of King Philip II of France) who in 1216 landed with an army, unopposed, on the Isle of Thanet in Kent. There was little resistance when the prince entered London and he was almost immediately proclaimed King Louis I of England at St Paul's Cathedral with great pomp and celebration in the presence of all of London. Although he was not yet crowned many English nobles as well as King Alexander II of Scotland on behalf of his possessions in England, gathered in London to give homage. Louis, with his army now boosted by numerous English lords, moved on to England's other capital Winchester, and then very soon after that and almost without a fight, he was in control of half of the kingdom. However just when it seemed that England was his, the hated king John suddenly died, which caused many of the rebellious barons to desert Louis in favour of John's nine-year-old son, another Henry, now proclaimed Henry III. Louis went back to France eventually becoming king Louis VIII, but of France alone. However it was another occasion when France and England could potentially have been united under a single monarch.

To that we could add the Empress Matilda, she was actually hailed as Queen after King Stephen had been captured in 1141; but, as with Louis, she was never crowned and back then a coronation secured the Crown not proclamation.

Ah yes, the Empress Matilda/Maude, although as you say she too never actually got the all important coronation despite King Stephen (who was then her prisoner) apparently being willing to renounce the throne. She very nearly got crowned but her coronation was called off at the very last moment (with only a day or so to go) when the citizens of London suddenly rose up against her -  in large part because of her haughty and arrogant attitude during the prelude to her coronation - and so she had to flee the city still uncrowned. Matilda never got a second chance and so while she physically possessed Stephen's crown (it was about all that was left of his jewels, the rest having been sold to pay for the war) she never got to legally wear it.

However what about Edward V? He legally succeeded on his father, Edward IV's death, but was under the age of majority, was never crowned, never had any independent power or influence, and died and his body disposed of in unknown circumstances just 3 or 4 months later. The Duke of Gloucester (his legal Protector) had him formally deposed so he could reign as King Richard III and this was legally confirmed at the time by the Titulus Regius Act. Yet despite all these moves to erase him from history, Edward is still recognised as a King of England with a regnal number. Is he accepted as being a true king simply because it was expedient for Henry Tudor to suppress the Titulus Regius - thereby restoring Edward's brief reign and so also to vilify Richard III - just to bolster his own claim to the throne?

Somewhat similar perhaps is the case of Lady Jane Grey. When Edward VI died he stipulated in his will that the crown should pass to her, which removed his half-sisters, Mary and Elizabeth, from the line of succession on account of their illegitimacy, thus subverting their claims under Henry VIII's Third Succession Act (1544). Jane was duly proclaimed queen on 10 July 1553 and preparations for her coronation started, but support for Mary (next in line according to Henry VIII's act, despite her illegitimacy) quickly grew and most of Jane's supporters abandoned her. The Privy Council of England, which had initially supported Jane's claim, suddenly changed sides and proclaimed Mary as queen on 19 July 1553, thus deposing Jane. Accordingly Jane was convicted of high treason in November 1553 and together with her husband was executed on 12 February 1554.

Unlike Edward V, Jane is never regarded as a true Queen of England, other than as the rather mocking "Nine Days Queen", however in many ways she was the true appointed heir to the throne, and might have been crowned as such had Mary (or rather her controllers/supporters) been slower off the mark and rather less forceful in reacting to events. As so often I guess it's the winners that get to write the history and so can then decide who is worthy of being remembered as a true monarch unfortunately let down by circumstance ... and who should be seen as a traitorous usurper that God never really intended should ever have reigned in the first place.

Many country's histories are littered with maybe monarchs, personally I consider Jane to have been Queen, although the legality was ambiguous to say the least. 
What about Sven (Sweyn) of Denmark? King by conquest in late 1013, never crowned only "accepted" as King, with some evidence that he was proclaimed as such on Christmas Day, was he really King of England?
If anyone wants to give themselves a headache try resolving the events in France in the first week of August 1830 - four Kings; Charles X, Louis XIX, Henri V and Louis Philippe if we accept the brief reigns of the middle two.

There’s also the case of Michael II of Russia. He was the younger brother of Tsar Nicholas II who abdicated in his favour on 15 March 1917. Michael was duly proclaimed Tsar the following morning. The thing was, however, that he hadn’t been informed and when he woke that morning he found that not only was he the new Tsar but he also had a meeting scheduled with members of the Provisional Committee of the State Duma. The meeting went on into the afternoon when a manifesto was agreed whereby Michael would not ‘assume the supreme power’ until a Constituent Assembly had been elected and convened to draft a new constitution. In the meantime the Provisional Committee would be recognised as the Provisional Government. The fact that the Provisional Government took its authority from Michael’s manifesto has been presented as proof that Michael was indeed head of state. 

Dating the end of Michael’s reign is tricky. It could be said that it ended with the issuing of his manifesto on the 16 March. Another date would be 5 months later when the Provisional Government declared Russia to be a ‘republic’ on 1 September. That declaration, however, was in direct contravention of the manifesto since the Constituent Assembly hadn’t been elected yet let alone convened. A final date would be 13 June 1918 when Michael was murdered by Bolsheviks in a wood near Perm. Thus he predeceased his brother Nicholas and nephew Alexei by a month.

Napoleon II, the Emperor who never was.

On two occasions, Napoleon Bonaparte abdicated, once in 1814 and again in 1815. Both times he abdicated in favour of his son, Napoleon Francois Joseph Charles Bonaparte. Both times, Napoleon Jnr was not allowed to take the throne. In 1814, the Coalition refused to recognise him, however in 1815 the French Provisional Government issued the following proclamation (part), on the 24th June:

Frenchmen!
Within the period of a few days, brilliant successes and dreadful reverses have marked your destinies.
A great sacrifice appeared necessary to your peace and that of the World; and Napoleon abdicated the Imperial Throne. His Abdication forms the termination of his political life. His Son is proclaimed.


On the 26th, the Government changed its' mind:


Paris, June 26, 1815.
The commission of government, upon the report of the minister of state, entrusted provisionally with the portfolio of the ministry of justice, decrees as follows: The decrees and judgements of the courts and tribunals, the acts of the notaries, shall provisionally be intituled, In the name of the French people. The minister of state, entrusted provisionally with the portfolio of the ministry of justice, is charged with the execution of this decree, which shall be inserted in the bulletin of the laws.
Note "in the name of the French people" after 3 days Napoleon II had been deposed.

The former Napoleon II lived the rest of his short life in Vienna, as Franz, Duke of Reichstadt, and became an officer in the Austrian Army. He seems to have been a conscientious student, who might have proved to be a capable officer, however he died of pneumonia aged 21. The succession passing to his cousin who did become Emperor as Napoleon III

There's also King Arthur - not the mythical one, nor the eldest son of Henry Tudor - but Arthur, son of Geoffrey of Brittany and a grandson of Henry II. As part of a treaty between Richard I (the Lionheart), Phillipe II of France and Tancred King of Sicily signed on 4 March 1191 at Messina, Arthur of Brittany had been designated heir to the throne of England and its French territories by Richard, the intent being that in the event of him having no legitimate heir of his own, the crown should pass to Arthur in preference to Richard's younger brother John. Richard didn't trust John, all the more so when a year later John led a rebellion against him when he was away on the Third Crusade. However when Richard died on 6 April 1199 (while fighting in France, aged 42) and still with no legitimate children of his own, Arthur was only twelve. The French territories supported Arthur's succession as King but the English nobles and royal advisors preferred Richard's brother John. Strictly by rules of primogeniture and international agreements Arthur should have become king but the royal council, claiming ancient rights under the old Anglo-Saxon principal of elective monarchy, got their man declared king instead (an action many of them probably came to regret). As it turned out young Arthur died only a few years later in about 1203 leaving no heirs himself, so it probably wouldn't have made that much difference in the long run if he had become king, but who can really say.

Shakespeare has King John killing Prince Arthur with his own hands. In reality Arthur was indeed murdered but not by the King himself.

I had assumed the lad just died of natural causes - always a dangerous assumption for the children of nobility in the Middle Ages. So if Arthur had become king in 1199 with the backing of the nobles (and presumably with a regent for a few years until he came of age) then his nasty uncle John may have been thwarted, with Arthur possibly going on to rule for many years leaving his own successors as the subsequent kings of England. That might have changed history a bit with possibly no Baron's war, no Magna Carta and maybe no loss of French territories.